(1) Field of the Invention
The present invention relates to a thin-film microstructure sensor in which a thin-film platinum temperature-sensitive resistor is formed on an insulation layer of a substrate. The thin-film microstructure sensor is suitably applied to a flow sensor, a humidity sensor, a gas sensor or a temperature sensor. The present invention further relates to a method of producing a thin-film platinum temperature-sensitive resistor on an insulation layer of a substrate through a sputtering process.
(2) Description of the Related Art
A thin-film microstructure sensor in which a thin-film platinum layer is provided on an isolated substrate for the purpose of measurement is known. The thin-film microstructure sensor of this type utilizes temperature-resistance characteristics of the thin-film platinum layer for measuring a flow rate of fluid, for measuring a humidity of an ambient atmosphere, for detecting presence of a gas in an ambient atmosphere, or for measuring a temperature of an ambient atmosphere. The thin-film microstructure sensor is capable of converting a change in resistance of the platinum layer into a change in temperature of the fluid or the ambient atmosphere. In the thin-film microstructure sensor, the measurement or the detection is performed based on the resistance-to-temperature conversion. The thin-film microstructure sensor is suitably applied to a flow sensor, a humidity sensor, a gas sensor or a temperature sensor.
For example, a conventional thin-film microstructure sensor uses a silicon substrate having an insulation layer, and a size of the substrate is on the order of a millimeter square. In the conventional thin-film microstructure sensor, a recessed portion is provided in the silicon substrate, and a bridge portion, including a heating element of platinum and two temperature-sensing elements of platinum, is provided over the recessed portion of the substrate. Some proposals for the arrangement of the bridge portion in the conventional thin-film microstructure sensor have been made in order for the heating element to efficiently generate heat and reduce heat loss to the substrate or to a supporting base which supports the substrate thereon.
A method of measuring a flow rate of a fluid used by the thin-film microstructure flow sensor will now be explained. That is, the thin-film microstructure flow sensor is placed into a flow of a fluid such that the temperature-sensing elements of the bridge portion extend in a direction perpendicular to the direction of the fluid flow. The temperature-sensing elements of the flow sensor are an upstream temperature-sensing element and a downstream temperature-sensing element with respect to the fluid flow. A control circuit of the flow sensor supplies voltage to the heating element and the upstream and downstream temperature-sensing elements of the bridge portion such that electric current flows through the bridge portion to increase a temperature of the bridge portion. The voltage supplied to the bridge portion is controlled by the control circuit such that both the bridge portion and the fluid are held at a constant temperature. The flow sensor under this condition measures a flow rate of the fluid based on a change in the voltage supplied to the bridge portion.
Japanese Laid-Open Patent Application No. 61-235726 discloses a flow measuring apparatus including a conventional thin-film microstructure flow sensor of the above-mentioned type. The flow measuring apparatus measures a flow rate of a specific gas used in a fabrication process of a semiconductor device. The flow measuring apparatus of the above-mentioned publication includes several thin-film microstructure sensors, such as a thin-film microstructure flow sensor provided in an internal passage of the semiconductor device, a stagnant gas flow sensor provided in a stagnant gas passage of the semiconductor device, and a temperature sensor provided at an inlet of the internal passage directed to the thin-film microstructure flow sensor. The temperature sensor is used to retain a temperature of the gas at the inlet of the internal passage.
Generally, accuracy of measurement of the thin-film microstructure sensors greatly depends on a temperature coefficient of resistance (TCR) of the platinum layer on the substrate. However, there is a problem in that the TCR of the platinum layer of the thin-film micro-structure sensors which are produced by manufacturing processes may vary greatly. The resistance-temperature characteristics of the sensor in operation are very sensitive to variations of the TCR of the platinum layer. The yield of the thin-film microstructure sensors providing a desired level of accuracy of measurement will be lowered due to the variations of the TCR, which will increase the cost of manufacture.
In addition, in the conventional thin-film microstructure sensors, such as flow sensors, humidity sensors and gas sensors, the heating element of the platinum layer is heated to an increased temperature above a room temperature. Repeated use of the conventional thin-film microstructure sensors over a prolonged time may cause migration in the platinum layer, and a value of resistance of the platinum layer tends to vary significantly. Therefore, there is a problem in that the accuracy of measurement of the conventional thin-film microstructure sensors is not reliable if the variations of the resistance value of the platinum layer exceed the order of several percents.